I'm just asking cause I see on a number of sites 18490/18500 batteries. So I wasn't sure of they were the same size or if they were just close enough in size to be interchangeable.
Generally speaking, they are interchangeable. What's important is to get the right
chemistry that you are looking for.
Protected ICR (Li-Ion) batteries will be longer (18500) due to the added protected circuit on the positive end of the battery.
IMR (Li-Mn) batteries will be shorter (18490) because being a
safer chemistry they do not require the protected circuit that ICR batteries require. IMR batteries are also called "un-protected", but most importantly they are also "
high drain". These are the batteries you want to use in regulated (variable voltage) mods, mechanical mods which use a Kick, or when using a Rebuildable Atomizer.
The most popular 18490 battery is the
AW 18490 IMR 1100 mAh.
Deeper Understanding of Mod Batteries